Intermittent or continuous methods of inhibiting the corrosion of steel contained in concrete structures are described. The equipment necessary to effect these methods can be incorporated into the structure during construction or retrofitted to existing structures. Cathodic protection systems are routinely used in the art, and it is known that electroosmosis will change the concentration of ions in the environment subjected to sufficient current to generate the electroosmotic effect. By xe2x80x9celectroosmotic effectxe2x80x9d is meant the movement of ions in water along the surface of solid concrete particles in a concrete structure.
This application is directed to a system which combines electroosmotic removal of corrosive anions from concrete and the cathodic protection of metal members embedded in concrete, such as in footings of steel bridges, the bases of communications towers, and more particularly, to the protection of reinforcing concrete members referred to as xe2x80x9crebarsxe2x80x9d in conventionally reinforced concrete structures. Such rebars are produced from mild steel (also referred to as xe2x80x9cblack steelxe2x80x9d) which has less than 1% carbon and less than 2% of alloying elements, combined. Removal of ions such as chlorides was taught by Slater, J. E. in an article titled Electrochemical Removal of Chlorides from Concrete Bridge Decks in xe2x80x9cMaterials Performancexe2x80x9d November 1976, pp 21-26. An electric field was applied between the reinforcement and an electrolyte on the concrete surface with the reinforcement as the negative pole. The chloride ions migrate through the concrete and either react with the electrolyte or are oxidized at the anode to chlorine gas which is evolved. Cathodic protection is typically effected either with (a) sacrificial anodes, or (b) impressed current with (i) potential control or (ii) current control, the reinforcement being the reactive cathode and the anode being substantially inert. Contamination of the concrete results in reaction of the cathode with the contaminants, and of course there is oxidation of the steel.
Typically reinforced steel structures such as bridges, buildings including power stations, marine structures such as docks, and roadways which are freshly built are most preferably immediately cathodically protected with an impressed current. But aged, internally reinforced and/or prestressed concrete structures which have been damaged because of chemical reaction with acidic elements in the ambient atmosphere cannot be adequately protected without first counteracting or eliminating the source of the problem causing the damage. The problem of protecting aged reinforced concrete structures is markedly different from cathodically protecting newly embedded rebars and other metal members in a concrete structure.
Though electroosmotic removal of corrosive anions from within aged and contaminated concrete, and, cathodic protection with either a sacrificial anode or an impressed current are routinely practiced, the effect of first using an electroosmotic current to deplete corrosive ions in concrete, then protecting the reinforcing members in the anion-depleted concrete with an impressed cathodic current was never considered. Neither was it considered to first use an electroosmotic current to deplete the corrosive ions, then without shutting off the electroosmotic current, concurrently providing an impressed cathodic current to protect the reinforcing members.
Improvements in the basic Slater process have been disclosed in U.S. Pat. Nos. 4,823,803; 4,865,702; 5,141,607; 5,228,959; inter alia. Electroosmotic current has also been used in porous concrete or masonry building materials, to transport water out of the material to minimize damage due to moisture. Typical of the technology dealing with concern about moisture in such materials, U.S. Pat. No. 6,126,802 teaches that the process comes to a stop due to the build-up of a potential on the electrodes. Thus the conditions under which direct current is applied to the material being treated, and apparently minor differences in composition and condition of the material being treated have a disproportionately large effect on the results of the treatment. The references do not suggest that for electroosmotic removal of corrosive anions the reinforcing members need not be the cathode, and that the electroosmotic current effectively depletes the anions in the concrete even when the electrolyte is a saline solution which is substantially pH neutral (pH 7-8); nor do the references suggest that, when the reinforcing members within the concrete are not used as the cathode, direct current usage is comparatively much lower; further, that as the ions within contaminated concrete are removed, it is unnecessary either to take core samples of the concrete, or, to analyze the electrolyte to analyze for the remaining corrosive ionic content of the concrete; moreover, there is no observed build-up of a potential on the electrodes and no pulsing required.
A system is provided for controlling corrosion of reinforced concrete which is contaminated with atmospheric pollutants such as sulfur oxides, nitrogen oxides, hydrogen sulfide, and road treatment salts such as sodium chloride and potassium chloride, all of which permeate the concrete structure and attack the steel rebars. This invention combines either (a) electroosmotic treatment with cathodic protection using a sacrificial anode, or, (b) electroosmotic treatment with cathodic protection using an impressed current. The former removes ions detrimental to steel and reduces the corrosivity of the environment surrounding the steel.
Since electroosmosis depletes the concentration of ions in the concrete environment thus increasing the resistivity of the concrete, it would be logical to conclude that under such conditions the current required to maintain cathodic protection would increase; eventually the conductivity would be so low that the current density for cathodic current would be uneconomical and have to be discontinued. Therefore it was not evident that subjecting the reinforced concrete to an electroosmosis treatment would be likely to decrease the power requirements for maintaining adequate corrosion protection of the rebars.
To provide a basis for comparing the effect of combining processes in which the conditions are different, efficiency of the processes to combat corrosion is used as a common parameter. xe2x80x9cEfficiencyxe2x80x9d is stated as being zero when there is no protection of any kind; efficiency is defined as the amount of metal which was not lost because of protection, divided by the amount of metal which would be lost with no protection, or:
(corrosion rate with no protection)xe2x88x92(corrosion rate with protection) divided by (corrosion rate with no protection).
The following terms are used in this disclosure:
xe2x80x9cEcxe2x80x9d refers to the corrosion potential of the rebar. Ec is measured with a reference electrode placed in contact with the circumferential surface of the concrete sample. It is written negative relative to a standard hydrogen electrode.
xe2x80x9cEpxe2x80x9d refers to the potential at which an effective impressed current for cathodic protection is to be supplied.
xe2x80x9cCDxe2x80x9d: current density for cathodic protection=current divided by the superficial area of the rebar in contact with concrete.
xe2x80x9cCPxe2x80x9d: impressed current for cathodic protection, identified separately when different.
xe2x80x9cEPxe2x80x9d: direct current for electroosmotic treatment which removes contaminant anions from the concrete;
xe2x80x9cELxe2x80x9d refers to an aggressive, substantially neutral pH, saline solution which serves as electrolyte in which samples are immersed.
It has been discovered that combining an electroosmosis direct current (EP) applied at less than 1 mA/Mcm3 (milliamp/1000 cm3 of concrete), preferably less than 0.2 mA/Mcm3 and voltage safe for humans, with the anode placed adjacent an outer surface of concrete soaked in a substantially neutral saline solution, effectively depletes corrosive anions in the concrete even when the direct current is in the range from 0.01 mA to less than 1 mA and at a voltage less than 100 V, preferably less than 70 V. Further, using such electroosmotic treatment as a first treatment until flow of current indicates depletion of harmful anions, and promptly, within less than six months, following the first treatment with cathodic protection, preferably by an impressed cathodic current (CP) at a comparably low voltage, the current density of CP required for cathodic protection is unexpectedly reduced. This decrease in the required current density of impressed current CP, coupled with low installation and operational costs of the novel system, improves the efficiency of a conventional cathodic protection system, whether by impressed current or with sacrificial anodes, several fold, as high as by a factor of 3 to 30 times. Moreover, though the electroosmotic treatment may be provided using the reinforcing members in the concrete as cathode, it is preferred to use a cathode outside the concrete structure; this xe2x80x9cexternalxe2x80x9d cathode for electroosmotic current (EP) is not the reinforcing members in the concrete.
It is therefore a general object of this invention to provide an electroosmotic treatment in combination with a cathodic protection system to enable one to maintain an aged ion-contaminated concrete structure essentially corrosion-free, using only a fraction of the current which would be required to maintain the same level of protection in a conventional cathodic protection system. Sequentially causing the electromigratory movement of contaminant ions out of the concrete, followed promptly by cathodic protection, and repeating the sequence as needed is effective. Concurrently providing both electroosmotic treatment and cathodic protection is unexpectedly even more efficient than sequential treatment, one circuit operating without substantially interfering with the other.
It is a specific object of the invention to provide a method for sequentially protecting, with separate electroosmotic and cathodic protection circuits, structures which are badly damaged due to the ravages of time in an acidic atmosphere. Electroosmotic treatment is commenced when resistance to direct EP current is low enough to allow more than about 1000 xcexcA/Mcm3 to flow at 36 V. EP is turned off when the current flow decreases to about 200 xcexcA/Mcm3 which indicates that the concentration of ions has dropped to an acceptably low level. The impressed cathodic current CP is turned on at a safe level of less than 100 V to maintain a potential Ep at a desired level, typically in the range from about 150 mV to less than 300 mV higher (numerically, though written as negative volts relative to a hydrogen electrode) than the corrosion potential of the rebars. CP is maintained until the current density rises above a level deemed economical. For example, when the current density rises above about 300 mA/m2 the costs of cathodic protection are generally deemed uneconomical; operation is preferably with current density of CP not above 200 mA/m2. CP is turned off when it is deemed uneconomical, and the circuit for electroosmotic treatment is then reactivated until enough ions are removed to make cathodic protection with impressed current CP alone, economical. This alternating sequence may be repeated as often as necessary to keep corrosion of the metal to a tolerable minimum over an indefinite period of time. The concentration of salts in the concrete is sensed by measurement of the current density required at a chosen safe voltage, and no analysis is required to determine the content of ions remaining in the concrete. Control of the system is effected with a programmable control means associated with a power source.
Alternatively, the electroosmotic treatment and cathodic protection of a chlorinated and sulfonated concrete structure may be commenced substantially concurrently by providing two separate electrical circuits which operate concurrently with separate anodes and cathodes until the levels of the electroosmotic current and the impressed cathodic current are too high to be economical. Thereafter only cathodic protection using either a sacrificial anode, or, an impressed current having lower current density, is necessary for adequate cathodic protection.